The present invention generally relates to Code Division Multiple Access (CDMA) communications techniques in radio telephone communication systems and, more particularly, to the demodulation of CDMA signals.
CDMA and spread spectrum communications have been around since the days of World War II. Early applications were predominantly military oriented. However, today there has been an increasing interest in using spread spectrum systems in commercial applications, including digital cellular radio, land mobile radio, and indoor and outdoor personal communication networks.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in cellular systems is to change from analog to digital transmission. Equally important is the choice of an effective digital transmission scheme for implementing the next generation cellular technology. Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-size, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by cellular carriers using the next generation digital cellular system infrastructure. The key feature demanded in these new systems is increased traffic capacity.
Currently, channel access is achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. System capacity is limited by the available frequencies as well as by limitations imposed by channel reuse. In TDMA systems, a channel consists of a time slot in a periodic train of time intervals over the same frequency. System capacity is limited by the available time slots as well as by limitations imposed on channel reuse.
With FDMA or TDMA or hybrid FDMA/TDMA systems, the goal is to ensure that two potentially interfering signals do not occupy the same frequency at the same time. In contrast, Code Division Multiple Access (CDMA) allows signals to overlap in both time and frequency. Thus, all CDMA signals share the same frequency spectrum. In the frequency or the time domain, the multiple access signals appear to overlap one another.
There are a number of advantages associated with CDMA communication techniques. The capacity limits of CDMA-based cellular systems are projected to be up to twenty times that of existing analog technology as a result of the properties of a wide band CDMA system, such as improved coding gain/modulation density, voice activity gating, sectorization and reuse of the same spectrum in every cell. CDMA transmission of voice by a high bit rate decoder ensures superior, realistic voice quality. CDMA also provides for variable data rates allowing many different grades of voice quality to be offered. The scrambled signal format of CDMA completely eliminates cross talk and makes it very difficult and costly to eavesdrop or track calls, ensuring greater privacy for callers and greater immunity from air time fraud.
In a "traditional" direct-sequence CDMA system, the informational data stream to be transmitted is impressed upon a much higher rate data stream known as a signature sequence to generate a transmitted sequence. The informational data stream and the high bit rate signature sequence stream are combined by effectively multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or -1. The informational data stream may consist of M'ary complex symbol values instead of binary +1 or -1 values. This combination of the higher bit rate signal with the lower bit rate data stream is called coding or spreading the informational data stream signal. Each informational data stream or channel is allocated a unique signature sequence.
Typically, the signature sequence data are binary, giving rise to stream of bits referred to as "chips." One way to generate this signature sequence is with a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. It is common for the period of the signature sequence to occupy one data symbol period, so that each data symbol is spread by the same Nc-chip signature sequence. In general, this signature sequence may be represented by real and imaginary numbers, corresponding to sending a chip value on the carrier frequency (I channel) or on a 90-degree shifted version of the carrier frequency (Q channel). Also, the signature sequence may be a composite of two sequences, where one of these sequences is a Walsh-Hadamard code word.
Typically the data symbols are binary. Thus, transmission of the signature sequence or its inverse represents one bit of information. In general, to send information symbol b using signature sequence s(n), one transmits EQU t(n)=b s(n) (1)
The receiver correlates the received signal with the known signature sequence to produce a detection statistic, which is used to detect b. For binary information symbols, when a large positive correlation results, a "0" is detected; when a large negative correlation results, a "1" is detected.
A plurality of coded information signals modulate a radio frequency carrier, for example by phase shift keying (PSK), and are jointly received as a composite signal at the receiver. Each of the spread signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. If the receiver is authorized, then the composite signal is correlated with one of the unique signature sequences, and the corresponding information signal can be isolated and decoded.
In the above example, a dam symbol b directly modulates a signature sequence s(n), which is commonly referred to as coherent modulation. The data symbol can be binary (+1 or -1), quaternary (+1, +j, -1, -j), or, in general, M'ary, taking on any of M possible values. This allows log.sub.2 (M) information bits to be represented by one information symbol b. In another traditional CDMA modulation scheme, the information is contained in how b changes from one symbol to the next, this being referred to as differentially coherent modulation. In this case, the true information is usually given by b(t) b*(t-Ts), where * denotes complex conjugation, t is a time index, and Ts is the information symbol period. In yet another traditional CDMA modulation scheme, sometimes referred to as noncoherent modulation, an M'ary information symbol is conveyed by transmitting one of M different signature sequences.
Another CDMA technique, called "enhanced CDMA", also allows each transmitted sequence to represent more .than one bit of information. A set of code words, typically orthogonal code words or bi-orthogonal code words, is used to code a group of information bits into a much longer code sequence or code symbol. A signature sequence is used to scramble the binary code sequence before transmission. This can be done by modulo-2 addition of the two binary sequences. At the receiver, the known scramble mask is used to descramble the received signal, which is then correlated to all possible code words. The code word with the largest correlation value indicates which code word was most likely sent, indicating which information bits were most likely sent. One common orthogonal code is the Walsh-Hadamard (WH) code. Enhanced CDMA can be viewed as a special case of noncoherent modulation.
In both traditional and enhanced CDMA, the "information bits" or "information symbols" referred to above can also be coded bits or symbols, where the code used is a block or convolutional code. One or more information bits can form a data symbol. Also, the signature sequence or scramble mask can be much longer than a single code sequence, in which case a subsequence of the signature sequence or scramble mask is added to the code sequence.
In many radio communication systems, the received signal includes two components: an I (in-phase) component and a Q (quadrature) component. This results because the transmitted signal has two components, and/or the intervening channel or lack of coherent carrier reference causes the transmitted signal to be divided into I and Q components. In a typical receiver using digital signal processing, the received I and Q component signals are sampled every Tc seconds, where Tc is the duration of a chip, and stored.
In mobile communication systems, signals transmitted between base and mobile stations typically suffer from echo distortion or time dispersion, caused by, for example, signal reflections from large buildings or nearby mountain ranges. Multipath dispersion occurs when a signal proceeds to the receiver along not one but many paths so that the receiver receives many echoes having different and randomly varying delays and amplitudes. Thus, when multipath time dispersion is present in a CDMA system, the receiver receives a composite signal of multiple versions of the transmitted symbol that have propagated along different paths (referred to as "rays") usually having relative time delays of less than one symbol period. Each distinguishable ray has a certain time of arrival k Tc seconds relative to the arrival of the first ray. If t(n) denotes the transmitted chip samples and r(n) denotes the received chip samples, where n is the discrete time index, then multipath time dispersion can be modeled as: ##EQU1## where Nr is the number of rays caused by the multipath dispersion.
As a result of multipath time dispersion, the correlator outputs several smaller spikes rather than one large spike. To detect the transmitted symbols (and recover the data bits), the spikes received are combined in some way. Typically, this is done by a RAKE receiver, which is so named because it "rakes" all the multipath contributions together using a weighted sum.
A RAKE receiver uses a form of diversity combining to collect the signal energy from the various received signal paths, i.e., the various signal rays. Diversity provides redundant communication channels so that when some channels fade, communication is still possible over non-fading channels. A coherent CDMA RAKE receiver combats fading by detecting the echo signals individually using a correlation method and adding them algebraically (with the same sign).
In one form of the RAKE receiver, correlation values of the signature sequence with the received signals at different time delays are passed through a tapped delay line. The values stored in the delay line are weighted and then summed to form the combiner output. When the earliest arriving ray correlation is at one end of the tapped delay line and the latest arriving ray correlation is at the other end of the tapped delay line, the weighted sum is selected to give the combined signal value for a particular information symbol period. This is effectively sampling the output of a complex FIR filter, whose coefficients are the weights which are referred to as the RAKE tap coefficients. Usually only the real part of the filtered value is used. Also, in some implementations, only the selected filter output is actually computed.
A diagram of a conventional RAKE receiver using post-correlator coherent combining of different rays is shown in FIG. 1. A received radio signal is mixed down to baseband and sampled, for example, by mixing it with cosine and sine waveforms and filtering the signal in an RF receiver 100, yielding I and Q chip samples. These chip samples are correlated to the known signature sequence in the correlator 101. Correlation values are then filtered by a finite-impulse-response (FIR) filter 102, which combines correlation values together using complex weights corresponding to the channel tap coefficients. Sometimes only the real part of the weighted values is needed. For example, if binary coherent modulation is used, then the sign of the real part of the selected value indicates whether a "+1" or "-1" was sent. At the appropriate time, based on symbol timing information, the FIR filter output is selected by selector 103, whose output is provided to a thresholding device 104, which uses the selected value to determine the information symbol. A channel tracking unit 105 is used to estimate the channel tap coefficients for the FIR filter 102.
Mathematically, suppose r(n)=I(n)+jQ(n) are the received chip samples, where I(n) are the I component samples, Q(n) are the Q component samples, and n is the sample index (discrete time index). The correlator correlates these data to the known signature sequence, s(n), to produce ##EQU2## where superscript * denotes complex conjugation, which is only needed if the signature sequence is complex.
The RAKE combiner is then a FIR filter that filters the correlations to produce a detection statistic z for transmitted symbol b. ##EQU3## where the filter coefficients a(k) are chosen as the channel tap coefficients: EQU a(k)=c(k) (5)
In practice, these would be channel tap coefficient estimates. In the case of binary modulation, only the real part of z is used.
Typically, the RAKE receiver has a limited number of taps, allowing it to process a limited number of rays. The taps do not need to be placed next to each other (e.g., if c(0), c(2), and c(5) are nonzero, these rays can be processed by a 3-tap RAKE receiver). However, in explaining the RAKE operation, it is convenient to assume that the tap locations are contiguous. The noncontiguous tap case is a special case of the contiguous tap case, where certain intervening taps have RAKE tap coefficients of zero. For example, a 3-tap RAKE which collects rays k=0, 2, and 5 is a special case of a 6-tap RAKE with collects rays at k=0 through 5, but with zero RAKE tap coefficients for rays k=1, k=3 and k=4.
The RAKE tap coefficient values given by equation (5) are based on the assumption that the spread-spectrum signal is received in the presence of white noise. White noise gives noise samples (chip samples) that are uncorrelated with each other.
In many systems, such as cellular systems, the receiver experiences interference from multiple transmitters, including the transmitter that transmits the desired signal. Also, noise from the environment affects receiver performance. Thus, in general, there are two sources of noise at the receiver: a) pre-channel noise, such as interfering signals from the same transmitter as the signal, and b) post-channel noise, including both thermal noise and interference from other transmitters. Pre-channel noise at the transmitter and post-channel noise at the receiver can often be modeled as white noise processes.
First consider the pre-channel noise. In most wireless CDMA applications, such as cellular communications, a star network is used in which mobiles communicate with a central structure referred to as a base station. In the downlink, also referred to as the forward path, the base station communicates with the mobiles by transmitting all signals simultaneously. Thus, at a particular mobile receiver, both the desired signal and interfering signals from that base station pass through the same channel. Assuming the interference can be modeled as white noise at the transmitter, then this interference is colored by the channel, giving rise to colored noise at the receiver. Consequently, part of the receiver noise is colored. In cellular systems, this part represents a large portion of the total noise.
The conventional RAKE filter was designed assuming white noise and does not work well when the noise is colored. Accordingly, the conventional RAKE filter is not an optimal solution for a mobile receiver. Thus, there is a need for a better downlink receiver for mobile units in radiocommunication systems.